In the majority of applications printing proceeds by pressure contact of an ink-loaden printing form with an ink-receiving material which is usually plain paper. The most frequently used impact printing technique is known as lithographic printing based on the selective acceptance of oleophilic ink on a suitable receptor. In recent times however so-called non-impact printing systems have replaced classical pressure-contact printing to some extent for specific applications. A survey is given e.g. in the book “Principles of Non Impact Printing” by Jerome L. Johnson (1986), Palatino Press, Irvine, Calif. 92715, USA.
Among non-impact printing techniques ink jet printing has become a popular technique because of its simplicity, convenience and low cost. Especially in those instances where a limited edition of the printed matter is needed ink jet printing has become a technology of choice. A recent survey on progress and trends in ink jet printing technology is given by Hue P. Le in Journal of Imaging Science and Technology Vol. 42 (1), January/February 1998.
In ink jet printing tiny drops of ink fluid are projected directly onto an ink receptor surface without physical contact between the printing device and the receptor. The printing device stores the printing data electronically and controls a mechanism for ejecting the drops image-wise. Printing is accomplished by moving the print head across the paper or vice versa. Early patents on ink jet printers include U.S. Pat. Nos. 3,739,393, 3,805,273 and 3,891,121.
The jetting of the ink droplets can be performed in several different ways. In a first type of process a continuous droplet stream is created by applying a pressure wave pattern. This process is known as continuous ink jet printing. In a first embodiment the droplet stream is divided into droplets that are electrostatically charged, deflected and recollected, and into droplets that remain uncharged, continue their way undeflected, and form the image. Alternatively, the charged deflected stream forms the image and the uncharged undeflected jet is recollected. In this variant of continuous ink jet printing several jets are deflected to a different degree and thus record the image (multideflection system). According to a second process the ink droplets can be created “on demand” (“DOD” or “drop on demand” method) whereby the printing device ejects the droplets only when they are used in imaging on a receiver thereby avoiding the complexity of drop charging, deflection hardware, and ink recollection. In drop-on-demand the ink droplet can be formed by means of a pressure wave created by a mechanical motion of a piezoelectric transducer (so-called “piezo method”), or by means of discrete thermal pushes (so-called “bubble jet” method, or “thermal jet” method).
Ink compositions for ink jet typically include following ingredients: dyes or pigments, water and/or organic solvents, humectants such as glycols, detergents, thickeners, polymeric binders, preservatives, etc. It will be readily understood that the optimal composition of such an ink is dependent on the ink jetting method used and on the nature of the substrate to be printed. The ink compositions can be roughly divided in:                water based; the drying mechanism involves absorption, penetration and evaporation;        oil based; the drying involves absorption and penetration;        solvent based; the drying mechanism involves primarely evaporation;        hot melt or phase change: the ink vehicle is liquid at the ejection temperature but solid at room temperature; drying is replaced by solidification;        UV-curable; drying is replaced by polymerization.        
It is known that the ink-receiving layers in ink-jet recording elements must meet different stringent requirements:                The ink-receiving layer should have a high ink absorbing capacity, so that the dots will not flow out and will not be expanded more than is necessary to obtain a high optical density.        The ink-receiving layer should have a high ink absorbing speed (short ink drying time) so that the ink droplets will not feather if smeared immediately after applying.        The ink dots that are applied to the ink-receiving layer should be substantially round in shape and smooth at their peripheries. The dot diameter must be constant and accurately controlled.        The receiving layer must be readily wetted so that there is no “puddling”, i.e. coalescence of adjacent ink dots, and an earlier absorbed ink drop should not show any “bleeding”, i.e. overlap with neighbouring or later placed dots.        Transparent ink-jet recording elements must have a low haze-value and be excellent in transmittance properties.        After being printed the image must have a good resistance regarding water-fastness, light-fastness, and good endurance under severe conditions of temperature and humidity.        The ink jet recording element may not show any curl or sticky behaviour if stacked before or after being printed.        The ink jet recording element must be able to move smoothly through different types of printers.        
All these properties are often in a relation of trade-off. It is difficult to satisfy them all at the same time.
When the finished ink jet image is meant to be transparent, e.g. for use in overhead projection or for use in a medical diagnostic hardcopy, it is necessary to use a transparent polymeric film (which may contain a colorant depending on the application) as support for the ink jet recording element. A well-known polymeric film for use as support in many fields of technology and especially in different types of image recording is a polyester, such as polyethylene terephthalate (PET) and polyethylene naphthalate (PEN). A problem when using such supports for ink receiving layers is the criticallity of the so-called “adhesion dry” property. This means that on further handling of the finished ink jet image, e.g. by preparing it for use as a master for the exposure of a printing plate, the ink receiver layer tends to get loose from the polyester support. When this phenomenon occurs in several areas the finished image gets a crumpled outlook and becomes unsuitable for use as a master, or parts of the image get lost which results in a loss of valuable information. In photographic industry where PET is widely used as support for photographic film the problem of the bad adhesion between the hydrophobic PET and the hydrophilic light-sensitive layer(s) is solved by providing the PET support with at least one and preferably two so-called subbing layers. However, it was found experimentally that this measure did not solve completely the problem of insufficient “adhesion dry” when typical ink receiver compositions are coated on a polyester support.